The present disclosure relates to an electrolytic solution with high crossover blocking performance suitable for electrochemical devices, such as direct methanol fuel cells (DMFCs), and an electrochemical device using the electrolytic solution.
There are energy density and power density as indicators showing the characteristics of a cell. The energy density is the amount of storage of energy per unit mass of the cell, and the power density is the amount of output per unit mass of the cell. Since lithium ion secondary batteries have two features of a comparatively high energy density and a very high power density, and also have a high degree of completeness, the lithium ion secondary batteries are often adopted as a power source of mobile devices. However, in recent years, the power consumption of the mobile devices tends to increase along with high performance. Further improvements in the energy density and power density are also required in the lithium ion secondary batteries.
The solutions to the problem include changes in electrode materials which constitute a positive electrode and a negative electrode, an improvement in a coating method of the electrode materials, an improvement in an encapsulating method of the electrode materials, etc., and researches which improve the energy density of the lithium ion secondary batteries are being undertaken. However, hurdles against practical use are still high. Additionally, unless constituent materials used for current lithium ion secondary battery change, an improvement in large energy density cannot be expected.
For this reason, the development of cells with a higher energy density to replace the lithium ion secondary battery is urgently needed, and fuel cells are expected as one amongst strong candidates.
The fuel cells have a configuration in which an electrolyte is arranged between a fuel electrode (anode) and an oxygen electrode (cathode), fuel is supplied toward the fuel electrode, and air or oxygen is supplied toward the oxygen electrode. As a result, in the fuel electrode (anode) and cathode (oxygen electrode), an oxidation reduction reaction which is oxidized by oxygen is caused, a portion of the chemical energy of the fuel is converted into electrical energy, and is taken out.
Various kinds of fuel cells are already suggested or manufactured by way of trial, and some of the fuel cells are put into practical use. These fuel cells are classified into alkali electrolyte fuel cells (AFCs), phosphoric acid fuel cells (PAFCs), molten carbonate fuel cells (MCFCs), solid oxide fuel cells (SOFCs), proton exchange fuel cells (PEFCs), etc. according to electrolytes used. Among these, the proton exchange fuel cells (PEFCs) have an advantage that the fuel cells can be operated at low temperature, for example, a temperature of 30° C. to about 130° C., compared with fuel cells of other types.
Various combustible substances, such as hydrogen and methanol, can be used as fuels of the fuel cells. However, since cylinders, etc, for storage are needed, gaseous fuels, such as hydrogen, are not suitable for miniaturization. On the other hand, liquid fuels, such as methanol, have an advantage that the fuels are easily stored. Especially, the direct methanol fuel cells (DMFCs) which directly supply methanol to and make it react with the fuel electrode have advantages such that a reformer for outputting hydrogen from fuel is not needed, a configuration becomes simple, and miniaturization is easy. Conventionally, many DMFCs are combined with PEFCs, and have been studied as one kind of PEFC.
In DMFCs, generally, methanol as fuel is supplied to the fuel electrode as a low-concentration or high-concentration aqueous solution, or pure methanol is supplied to the fuel electrode in a gas state. Then, the methanol is oxidized into carbon dioxide in a catalyst layer of the fuel electrode. The protons produced at this time move toward the oxygen electrode through an electrolyte membrane which separates the fuel electrode from the oxygen electrode, react with oxygen in the oxygen electrode to produce water. The reactions which occur in the fuel electrode, an oxygen electrode, and the whole DMFC are expressed with the following reaction formulas, respectively.Fuel electrode: CH3OH+H2O→CO2+6e−+6H+Oxygen electrode: (3/2)O2+6e−+6H+→3H2OWhole DMFC: CH3OH+(3/2)O2→CO2+2H2O
The energy density of methanol that is fuel for DMFCs is theoretically 4.8 kW/L, and is 10 or more times the energy density of general lithium ion secondary batteries. That is, fuel cells using methanol as fuel have a great possibility of surpassing the energy density of the lithium ion secondary batteries. From this point, DMFCs among various fuel cells have a highest possibility of being used as an energy source of mobile devices, electric motorcars, etc.
However, DMFCs have a problem in that the output voltage when electricity is being actually generated may decrease to about 0.6 V or less even though the theoretical voltage is 1.23 V. One of the factors in the output voltage decrease is a voltage drop caused by the internal resistances of DMFCs. Internal resistances, such as resistance accompanying reactions caused in both electrodes, resistance accompanying the movement of substances, resistance caused when protons move through an electrolyte membrane, and contact resistance, exist inside DMFCs. Since the energy which can actually be output from oxidation of methanol as electrical energy is expressed by the product of an output voltage during power generation, and the quantity of electricity which flows through a circuit, when the output voltage during power generation decreases, the energy which can actually be output becomes small by that much. In addition, the quantity of electricity which can be output to the circuit by the oxidation of methanol is proportional to the amount of methanol in DMFCs, if the whole quantity of methanol is oxidized in the fuel electrode according to the above reaction formula.
Additionally, DMFCs have a problem of methanol crossover. The crossover is a phenomenon in which a reactant in one electrode (for example, fuel electrode) passes through an electrolyte membrane or an electrolytic solution, and reaches the other electrode (for example, oxygen electrode). The methanol crossover occurs according to two mechanisms including a phenomenon in which methanol diffuses and moves due to the concentration difference of methanol on the side of the fuel electrode and on the side of the oxygen electrode, and an electroosmotic phenomenon in which hydrated methanol is carried by the movement of water caused along with the movement of protons.
When the methanol crossover occurs, transmitted methanol is oxidized by a catalyst layer of the oxygen electrode. Although a methanol oxidation reaction on the side of the oxygen electrode is the same as an oxidation reaction on the side of the fuel electrode mentioned above, this becomes the cause of reducing the output voltage of DMFCs (for example, refer to “Description, Fuel Cell System”, Ohmsha, Ltd., pp. 66). Additionally, since methanol is not used for power generation on the side of the fuel electrode but is wasted on the side of the oxygen electrode, the quantity of electricity which can be output to the circuit decreases by that much. Additionally, since a catalyst on the side of the oxygen electrode is not a platinum-ruthenium (Pt—Ru) alloy catalyst but a platinum (Pt) catalyst, there is a disadvantage that carbon monoxide (CO) is adsorbed on the surface of the catalyst, and poisoning of the catalyst occurs.
As described above, DMFC has two problems including a voltage drop caused by the internal resistance and the methanol crossover and the waste of fuel caused by the methanol crossover, and these problems become factors which reduce the power generation efficiency of DMFCs. Thus, in order to increase the power generation efficiency of DMFCs, the research and development which improve the characteristics of materials which constitute DMFCs, and the research and development which optimize the operating conditions of DMFCs are energetically undertaken.
The research which improves the characteristics of materials which constitute DMFCs includes researches on an electrolyte membrane, a catalyst on the side of a fuel electrode, etc. Currently, a poly perfluoro alkyl sulfonic acid-based resin layer (for example, Nafion by E. I. du Pont de Nemours & Co. (registered trademark)) is generally used as the electrolyte membrane. However, as materials having higher proton conductivity and higher methanol loss crossover blocking performance than this resin layer, fluorine-based polymer membranes, hydrocarbon-based polymer electrolyte membranes, or hydro gel base electrolyte membranes are presently being studied. As for the catalyst on the side of the fuel electrode, research and development is being undertaken of the catalyst with higher activity than a platinum-ruthenium (Pt—Ru) alloy catalyst which is generally being used now.
Such an improvement in characteristics of constituent materials of the fuel cells is appropriate as a means for improving the power generation efficiency of the fuel cells. However, in the current situation, optimal electrolyte membranes and optimal catalysts that can solve the two problems of DMFCs mentioned above have not yet been discovered.
Thus, in Journal of the American Chemical Society, 2005, 127th volume, No. 48, pp. 16758 to 16759 (FIG. 1), and in US Patent Application Publication No. 2004-0072047 (4th and 5th pages, FIG. 7), laminar flow (streamline flow) fuel cells which do not use an electrolyte membrane are suggested. In the laminar flow fuel cells, problems such as flooding in an oxygen electrode, moisture management and crossover of fuel, are solved.
In the laminar flow fuel cells, a fuel electrode and the oxygen electrode are arranged at wall surfaces with micro flow passages, and an electrolytic solution flows through the micro flow passages. As the electrolytic solution, a liquid consisting of a fuel and an electrolytic solution are used on the side in contact with the fuel electrode, and a liquid consisting of an electrolytic solution including oxygen is used on the side in contact with the oxygen electrode. In addition, the oxygen electrode is porous, and if oxygen is supplied through the oxygen electrode from the surface of the oxygen electrode opposite to the micro flow passages, the electrolytic solution which flows in contact with the oxygen electrode does not need to include oxygen.
The two kinds of liquids mentioned above flow forming a laminar flow, and an interface is formed at the boundary between the two kinds of liquids. As a result, the two kinds of liquids are adapted so as not to be suddenly mixed together by the motion of a fluid at macroscopic scales. In the laminar flow fuel cells, continuous power generation is possible, as such two (or more) kinds of liquids form a laminar flow and circulate without being mixed together. Although the interface is formed at the boundary between the two kinds of liquids, since molecules and ions can pass through the interface freely by a micro diffusion motion, electrochemical connection between the two kinds of liquids is maintained.
As described above, since an electrolyte membrane is not used in the laminar flow fuel cells, the problems caused by the proton-exchange fuel cell (PEFC) using an electrolyte membrane, for example, the problems of the electrolyte membrane deteriorating due to secular change, proton conductivity decreasing due to drying (lack of moisture) of the electrolyte membrane caused by a temperature rise, and power generation efficiency decreasing, do not exist.
In addition, the conditions under which the flow of the electrolytic solution becomes a laminar flow include a case in which the Reynolds number is small. The Reynolds number is a ratio between inertial force and viscous force, and is defined by the following Expression (1). Generally, if the Reynolds number is less than 2000, flow becomes a laminar flow.Reynolds number=(Inertial force/Viscous force)=ρUL/μ=UL/ν  Expression (1)
In the above Expression, ρ is the density of a fluid, U is characteristic velocity, L is characteristic length, μ is a viscosity coefficient, and ν is kinematic viscosity.
Since an electrolyte membrane is not used in the laminar flow fuel cells shown in Journal of the American Chemical Society, 2005, 127th volume, No. 48, pp. 16758 to 16759 (FIG. 1), and in US Patent Application Publication No. 2004-0072047 (4th and 5th pages, FIG. 7), a problem resulting from the electrolyte membrane is not caused. Additionally, the flooding, etc. in the oxygen electrode etc. is solved.
However, in the laminar flow fuel cells configured as the direct methanol fuel cells (DMFCs), the interface is formed at the boundary between the two kinds of liquids. However, a wall which divides both the liquids does not exist. Thus, due to the concentration difference between methanol on the side of the fuel electrode and methanol on the side of the oxygen electrode, a portion of methanol diffuses and moves through the interface from the fuel electrode to the oxygen electrode, and methanol crossover occurs. Although the methanol crossover occurs regardless of the degree of fuel concentration, especially when a high-concentration methanol aqueous solution is used as fuel, the influence of the methanol crossover appears prominently due to a large concentration difference. This is the same even in a case where the two kinds of liquids are locally divided by a separator. For example, in Journal of the American Chemical Society, 2005, 127th volume, No. 48, pp. 16758 to 16759 (FIG. 1), an example is reported in which the flow of the two kinds of liquids is a laminar flow, the influence of the crossover appears reliably in a methanol concentration of about 8 mols in spite of putting the separator between electrodes, and power generation characteristics degrade.
Additionally, as described with reference to FIG. 5 in Embodiment 3 which will be described later, the amount of methanol crossover can be significantly reduced compared with DMFC suggested in Journal of the American Chemical Society, 2005, 127th volume, No. 48, pp. 16758 to 16759 (FIG. 1) by completely separating the liquid flow consisting of a fuel and an electrolytic solution, and the flow of the liquid consisting only of an electrolytic solution. However, even if fuel supply speed and fuel concentration are controlled as long as the electrolytic solution which can dissolve methanol between the fuel electrode and the oxygen electrode is arranged, it is impossible to eliminate the methanol crossover completely. Additionally, in a fuel cell which generates electricity by circulating an electrolytic solution, crossovered methanol is accumulated and increases with power generation time even if the amount of methanol crossover generated per unit time is little. Thus, it goes without saying that the methanol crossover becomes a problem.